Travel by aircraft is generally a safe and efficient way for travelers to reach remote destinations. Over the years, as the popularity of air travel has dramatically increased, the need for techniques for safely managing the flow of aircraft has also risen. To address air traffic safety issues, aircraft have been equipped with avionics equipment such as transponders that assist air traffic controllers in identifying, tracking, and managing aircraft in flight.
Through radio frequency transmissions, transponders provide air traffic controllers and other suitably equipped aircraft with information such as aircraft identification, altitude, and other aircraft-specific data. Ready access to such information allows controllers and pilots to utilize airspace in a safer and more efficient manner. As the density of air traffic grows, it is understandable that there is a growing need for more information to be relayed between aircraft and ground stations on a near-real-time basis.
Currently, FAA Air Traffic Control and most other ATC controlling authorities around the world use standard modulation schemes to ensure interoperability of their radio frequency signals with other aircraft and systems. For example, the Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne equipment, promulgated by RCTA as RTCA/DO-181C (and incorporated by reference herein in its entirety) defines pulse position modulation on 1090 MHz for Mode S transponder and older transponder (ATCRBS transponders) replies to 1030 MHz ground station and TCAS interrogations. By using standard protocols aircraft state information as well as other data can be relayed aircraft to ground, ground to aircraft, or in some instances aircraft to aircraft.
The volume of information that must be transmitted from aircraft continues to increase as more advanced avionics and traffic control systems become available. Likewise, the need to transmit diverse information of all kinds also drives the desire to utilize aircraft systems to send data. However, because of the large number of required transponder replies in heavily traveled areas (such as in the vicinity of an airport, where hundreds of replies per second are generated), there are worldwide limits on the number of transponder broadcast transmissions permitted each second from each aircraft. For example, the limit for Automatic Dependent Surveillance Broadcast (ADS-B) is currently set to 6.2 transmissions per second to prevent the additional ADS-B interference from potentially all the aircraft near a major airport creating a situation where the ATC ground station becomes unable to receive surveillance replies from aircraft in the terminal area being controlled by ATC.
ADS-B squitter data content has already been defined for the most part by industry committees such as SC186, and there is little remaining room for future growth. In fact, systems currently envisioned and being developed by avionics systems designers will likely need to transmit more data than can be sent within the 6.2 squitters per second limit. The ability to employ more data in avionic systems is now and will continue to be needed.
The existing Mode S transponder reply data format (also known as squitters when they are sent unsolicited by an interrogation) is implemented with a pulse position modulation technique, where the position of a pulse determines whether a bit is a one or a zero. Referring to the transmission reply data format and timing diagram 200 in FIG. 2, the first four pulses 203 within the 8 microsecond preamble time 210 are called preamble pulses and are used to determine that the pulse position data that follows is for a Mode S reply (or squitter). ADS-B squitters use the long Mode S reply format and thus contain 112 bits in the data block 220 per squitter. In other applications, 56 bits may be transmitted.
Data is transmitted through digital data encoded in the Data Block 220. A bit interval 202 comprises two sub-intervals defining the logical state of a bit symbol. When a pulse is in the “1” sub-interval position (FIG. 3, 301) of a bit interval 202, that bit value is a 1 and when a pulse is in the “0” sub-interval position (FIG. 3, 302) of a bit interval 202, that bit value is a 0. Only one pulse either in a “0” or a “1” position is permitted for each bit interval or bit symbol period (such as bit interval 202) of the entire message shown 200.
Referring to FIG. 3, an expanded view of bit interval 202 is shown. A carrier wave in the form of a pulse waveform 310 is being transmitted in the “1” position 301, and no pulse is transmitted 315 during the “0” sub interval, and therefore, this bit interval represents the value of logical “1.” Note that the sinusoidal waveform 310 provided in the drawing is for illustration purposes only, and as a standard frequency for ADS-B replies is currently 1090 MHz+/−1 MHz, approximately 545 cycles of the waveform 310 would normally occur during the 0.5 microsecond sub-interval 301. The phase of the waveform 310 is also unimportant for existing transponder reply standards. What is needed are methods and systems to make more efficient use of transponder reply messages to increase data throughput and provide for additional communication links.